Optical communication networks are often used to transport large amounts of data over long distances very quickly. At present, sophisticated optical communication networks are capable of transmitting tens of trillions of bits of information per second over a single optical fiber spanning many hundreds of kilometers. Optical communication networks generally exceed the bandwidth capabilities of copper networks. As a result, optical networks are often used to form undersea telecommunication networks and to form optical backbones in wireline telecommunication networks.
Optical communication networks can use lasers to pump optical amplifiers and to generate optical signals that are transported through the networks and through the amplifiers. For safety and other reasons, it is often necessary or desirable to detect a loss of signal (LOS) condition over an optical network communication link, which could be indicative of a cable break or other fault. Upon the detection of an LOS condition, the laser or lasers being used to communicate over that communication link can be shut down as part of an automatic laser shutdown (ALS) process. This can help to reduce or prevent injury to people and damage to equipment.
Unfortunately, conventional LOS detection techniques fall short in various ways. In some conventional approaches, a Raman amplifier module provides pump power that is multiplexed with optical signals onto a signal fiber, and a signal monitor attempts to detect a loss of signal power from the amplifier module over the signal fiber. However, amplified spontaneous emissions (ASE) generated by the amplifier module's pumps travel in the same direction as the optical signals and can be quite powerful. This makes detecting an LOS condition difficult or impossible in some cases.
In other conventional approaches, a residual pump monitor can measure the amount of residual pump power that is received over a signal fiber from a Raman amplifier module. However, in some cases, the amplifier module might not provide enough pump power into the signal fiber so that residual power is received by the residual pump monitor, or backscatter created by pumps within the amplifier module could be much higher than any residual pump power. Moreover, some conventional optical spans include remote optically pumped amplifiers (ROPAs), and isolators in the ROPAs can block residual pump power. Additionally, rare-earth doped fibers in the ROPAs may absorb the pump power. These factors can prevent the residual pump monitor from detecting an LOS condition.